In speech communications, encoding and decoding of the background noise are performed according to a noise processing scheme defined in G.729B released by the International Telecom Union (ITU).
A silence compression technology is introduced into a speech encoder, and FIG. 1 shows the schematic diagram of the signal processing.
The silence compression technology mainly includes three modules: Voice Activity Detection (VAD), Discontinuous Transmission (DTX), and Comfort Noise Generator (CNG). VAD and DTX are modules included in the encoder, and CNG is a module included in the decoding side. FIG. 1 is a schematic diagram showing the principle of a silence compression system, and the basic processes are as follows.
First, at the transmitting side (i.e., the encoding side), for each input signal frame, the VAD module analyzes and detects the current input signal frame, and detects whether a speech signal is contained in the current signal frame. If a speech signal is contained in the current signal frame, the current frame is marked as a speech frame. Otherwise, the current frame is set as a non-speech frame.
Then, the encoder encodes the current signal based on a VAD detection result. If the VAD detection result indicates a speech frame, the signal is input to a speech encoder for speech encoding and a speech frame is output. If the VAD detection result indicates a non-speech frame, the signal is input to the DTX module where a non-speech encoder is used for performing background noise processing and outputs a non-speech frame.
Finally, the received signal frame (including speech frames and non-speech frames) is decoded at the receiving side (the decoding side). If the received signal frame is a speech frame, it is decoded by a speech decoder. Otherwise, it is input to a CNG module, which decodes the background noise based on parameters transmitted in the non-speech frame. A comfort background noise or silence is generated so that the decoded signal sounds more natural and continuous.
By introducing such a variable bit-rate encoding scheme to the encoder and performing a suitable encoding on the signal of the silence phase, the silence compression technology effectively solves the problem that the background noise may be discontinuous and improves the quality of synthesized signal. Therefore, the background noise at the decoding side may also be referred to as comfort noise. Furthermore, the background noise encoding rate is much lower than the speech encoding rate, and thus the average encoding rate of the system is reduced substantially so that the bandwidth may be saved effectively.
In G.729B, signal processing is performed on a frame-by-frame basis. The length of a frame is 10 ms. To save bandwidth, G.729.1 further defines the silence compression system requirements. It is required that in the presence of the background noise, the system should encode and transmit the background noise at low bit-rate without reducing the overall signal encoding quality. In other words, DTX and CNG requirements are defined. More importantly, it is required that the DTX/CNG system should be compatible with G.729B. Although a G.729B based DTX/CNG system may be transplanted simply into a G.729.1 based system, two problems remain to be settled. First, the two encoders will process frames of different lengths, and thus direct transplantation may be problematic. Moreover, the 729B based DTX/CNG system is relatively simple, especially the parameter extraction part. To meet the requirements of DTX/CNG in G.729.1, the 729B based DTX/CNG system should be extended. Second, the G.729.1 based system can processes wideband signals but the G.729B based system can only process Lower-band signals. A scheme for processing the Higher-band components of the background noise signal (4000 Hz˜7000 Hz) should thus be added to the G.729.1 based DTX/CNG system so as to form a complete system.
The prior arts at least have problems as follows. The existing G.729B based systems can only process Lower-band background noise, and accordingly the signal encoding quality cannot be guaranteed when being transplanted into the G.729.1 based systems.